ISO 269 · Worldwide except North America · 7 sizes
ISO envelopes
ISO 269 envelopes are sized to A-series paper. DL (110 × 220 mm) holds A4 folded into thirds, C6/C5 (114 × 229 mm) is a roomier alternative for the same fold, and the B envelopes (B4, B5, B6) are outer sleeves that hold a C envelope of the same number.
From B6 envelope (125 × 176 mm) to DL (110 × 220 mm).
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Every size in the series
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| Size | Width × Height (mm) | Centimetres | Inches | Points | Pixels @300 dpi | Ratio |
|---|---|---|---|---|---|---|
| DL DIN Lang | 110 × 220 mm | 11 × 22 cm | 4.33 × 8.66 in | 312 × 624 pt | 1,299 × 2,598 px | 1:2 |
| C6/C5 C65 | 114 × 229 mm | 11.4 × 22.9 cm | 4.49 × 9.02 in | 323 × 649 pt | 1,346 × 2,705 px | 1:2 |
| C7/C6 | 81 × 162 mm | 8.1 × 16.2 cm | 3.19 × 6.38 in | 230 × 459 pt | 957 × 1,913 px | 1:2 |
| E4 Holds a C4 envelope or B4 sheet | 280 × 400 mm | 28 × 40 cm | 11.02 × 15.75 in | 794 × 1,134 pt | 3,307 × 4,724 px | 1:1.43 |
| B4 envelope | 250 × 353 mm | 25 × 35.3 cm | 9.84 × 13.9 in | 709 × 1,001 pt | 2,953 × 4,169 px | 1:√2 |
| B5 envelope | 176 × 250 mm | 17.6 × 25 cm | 6.93 × 9.84 in | 499 × 709 pt | 2,079 × 2,953 px | 1:√2 |
| B6 envelope | 125 × 176 mm | 12.5 × 17.6 cm | 4.92 × 6.93 in | 354 × 499 pt | 1,476 × 2,079 px | 1:√2 |
Have a measurement instead of a name? Paste it into the paper size finder — it reads 210 x 297, 8.5 x 11 in or 24x36 and names the sheet.
About this series
ISO 269 (1985) defines the envelope sizes used with ISO 216 paper. The C series handles unfolded and half-folded sheets and has its own section; this section covers the other ISO envelopes: DL, the C6/C5 hybrid, C7/C6, E4 and the B envelopes. They are the standard stock of every stationer in Europe, Asia, Africa, Australia and Latin America outside the Letter-paper countries.
DL is 110 × 220 mm (4.33 × 8.66 in). The name comes from DIN Lang, the German long format. An A4 sheet folded into thirds measures 210 × 99 mm and drops in with 5 mm to spare each way. It is the standard business envelope and the size window envelopes are made in.
C6/C5, also written C65, is 114 × 229 mm. It takes the same A4 tri-fold as DL but with more clearance, which matters for automatic inserting machines and for thicker packs. C7/C6 (81 × 162 mm) is a slim envelope for A4 folded into six or for A5 folded into thirds. E4 (280 × 400 mm), from the German standard DIN 678-1 rather than ISO 269, is the outer envelope that holds a C4 envelope or a B4 sheet flat.
The B envelopes are one step up. B4 (250 × 353 mm) holds a C4 envelope, B5 (176 × 250 mm) holds a C5 and B6 (125 × 176 mm) holds a C6. They are used when a sealed envelope needs to travel inside another, or when a pack is too thick for the C size.
A practical tip: if a DL mailing keeps jamming in the inserter, switch to C6/C5. The extra 4 mm of height and 9 mm of length is usually the difference between a clean run and a stopped machine.
Choosing an envelope
The C series is designed so that each envelope takes the matching A sheet flat: C4 holds A4, C5 holds A5, C6 holds A6. Fold the sheet and you drop two steps, so an A4 folded once goes in a C5 and folded into thirds goes in a DL.
DL is the business default across Europe, the equivalent of the No. 10 elsewhere, and it is the size window envelopes and inserting machines assume. C4 is the choice when a document must not be creased at all, such as certificates or contracts for signature.
The chain, in one line
A4 flat needs C4. A4 folded once needs C5. A4 folded twice, or into thirds, needs DL or C6. The envelope finder resolves any sheet, including the ones that do not follow the pattern.
Windows, seals and postal limits
Window position on DL envelopes is conventional rather than absolute, and it differs between countries, so confirm the specification before laying out the letter. Self-seal is standard for volume work; gummed flaps remain common for formal correspondence.
Postal pricing changes at thickness as well as size. In the UK the large-letter threshold is the one that catches people out: a C4 envelope is already a large letter, and a few sheets too many pushes it into the parcel band. Weigh a made-up sample before committing to a run, and see what a sheet of paper weighs for the arithmetic.
Questions about this series
What size is a DL envelope?
DL is 110 × 220 mm (4.33 × 8.66 in). It holds an A4 sheet folded into thirds (210 × 99 mm) and is the standard business envelope in ISO countries.
What is a C6/C5 envelope?
C6/C5, also called C65, is 114 × 229 mm. It takes A4 folded into thirds like a DL but with 4 mm more height and 9 mm more length, which makes it easier to insert by machine.
What envelope holds a C4 envelope?
A B4 envelope, 250 × 353 mm. In the same way a B5 envelope (176 × 250 mm) holds a C5, and a B6 envelope (125 × 176 mm) holds a C6.
What is an E4 envelope?
E4 is 280 × 400 mm. It is a large envelope that holds a C4 envelope (229 × 324 mm) or a B4 sheet (250 × 353 mm) flat, with room for thick document packs.
What envelope fits an A4 sheet?
C4 takes an A4 sheet flat. Folded once it fits a C5, and folded into thirds it fits a DL.
Do window envelopes have a standard window position?
Only by convention, and the convention differs between countries and suppliers. Ask for the window specification before laying out the letter rather than after, because moving an address block is cheap and reprinting envelopes is not.
Why are C sizes bigger than A sizes?
Because they have to hold them. Each C size is slightly larger than the A size of the same number, with the B series sitting between them for envelopes that need to hold a folder as well.
How heavy can a letter be before the price changes?
It depends on the carrier and the destination, and it changes with thickness as well as weight. Weigh a made-up sample, including the envelope, before printing a run.